If she flinches and squirms, she’ll wiggle her butt a little.
Perhaps that was the tease and provocation that led to this in the first place. That, and being over-confident that in that outfit, pulling her skirt up would be too awkward in public.
"...the girls are prepared in the reformatory’s punishment room, naked, lying on their backs on special benches, bound with their legs spread above their heads, shamefully showing the two holes usually hidden..."
If she flinches and squirms, she’ll wiggle her butt a little.
Perhaps that was the tease and provocation that led to this in the first place. That, and being over-confident that in that outfit, pulling her skirt up would be too awkward in public.